Hysteresis and Longitudinal Variation in Air–Water Temperature Relationships in the Han River Basin

We examined changes in water temperature in relation to air temperature in the two upper, two middle and lower reaches of the Han River basin, Republic of Korea. Air and water temperatures were monitored from 2020 to 2025, with analyses restricted to the May–October period of each year. TW exhibited pronounced spatial variability, whereas TA exhibited relatively low spatial variability. Hysteresis loops were analyzed separately for the warming (May–July) and cooling (August–October) phases using the air–water temperature relationship. Distinct phase-dependent patterns in air–water temperature differences were observed. Air–water temperature differences were greater in streams than in rivers, with values 4–5 °C higher during the warming phase and 4–6 °C lower during the cooling phase. Water temperature responses to air temperature showed phase-dependent spatial patterns, with regression slopes varying across stream and river systems and temperature elasticity decreasing from streams to rivers. Therefore, the results indicate that water temperature responses to air temperature vary both spatially and temporally, with sensitivity differing along the longitudinal gradient.